1. Field of the Invention
The present invention relates to a clamp. In particular, but not exclusively, the invention relates to a clamp for exerting a pushing or pulling force on a workpiece.
2. Discussion of the Known Art
An example of a clamp for exerting a pushing or pulling force on a workpiece is that sold by HMC-Brauer Limited under the model numbers CP1005 to CP1008. This clamp has a roller cam comprising a pair of rollers mounted on spur axles that engage helical tracks provided in the inner cylindrical surface of the clamp body. Rotating a handle causes the rollers to move along the helical tracks, which drives a clamping member in the axial direction of the clamp. The helical tracks include portions of steep and shallow pitch, which allow the clamping member to be brought rapidly into engagement with the workpiece before the clamping force is applied. The clamp is very quick and simple to apply, with a single movement of the handle providing both the initial fast travel of the clamping member and also the final clamping force.
The main problem with the clamp described above is that axles of the roller bearings may shear or be distorted if an excessive force is applied. The nominal maximum holding force of the clamp is therefore only 500 daN, which can easily be applied with a hand impact on the handle. In practice in the workshop, the clamps are often mistreated and a mallet may be used to increase the clamping force. This frequently leads to damage to the clamp.
The above mentioned problem is compounded by the fact that travel of the clamping member is limited by the length of the helical tracks. If the rollers reach the ends of the tracks before a sufficient clamping force has been applied, there will be a temptation to apply the clamp harder as there is no clear indication that the point of maximum travel has been reached. This is again likely to cause damage to the clamp.
A further disadvantage of the clamp is that it is relatively complicated and expensive to manufacture.